Improved fuel efficiency for vehicles having internal combustion engines can be achieved by using diesel engines or gasoline engines operated with an excess of oxygen over the amount necessary for complete combustion of the fuel. Such engines are said to run "lean" or on a "lean mixture." The increase in fuel economy, however, is offset by undesired pollution emissions, specifically in the form of oxides of nitrogen (NOx).
One method used to reduce NOx emissions from internal combustion engines is known as selective catalytic reduction (SCR). SCR, when used, for example, to reduce NOx emissions from a diesel engine, involves injecting an atomized reagent into the exhaust stream of the engine in relation to one or more selected engine operational parameters, such as exhaust gas temperature, engine rpm or engine load as measured by engine fuel flow, turbo boost pressure or exhaust NOx mass flow. The reagent/exhaust gas mixture is passed through a reactor containing a catalyst, such as, for example, activated carbon, or metals, such as platinum, vanadium or tungsten, which are capable of reducing the NOx concentration in the presence of the reagent. An SCR system of this type is disclosed in U.S. patent application Ser. No. 08/1831,209, issued as U.S. Pat. No. 5,976,475 hereby incorporated by reference.
An aqueous solution of urea is known to be an effective reagent in SCR systems for diesel engines but suffers several disadvantages. Urea is highly corrosive and tends to attack mechanical components of the SCR system, such as the injectors used to inject the urea mixture into the exhaust gas stream. Urea also tends to solidify upon prolonged exposure to elevated temperatures, such as encountered in diesel exhaust systems. Solidified urea tends to accumulate in the narrow passageways and orifice openings typically found in injectors. The solidified urea fouls moving parts of the injector and clogs any openings, thus, rendering the injector unusable.
Furthermore, if the urea mixture is not finely atomized, urea deposits will form in the catalytic reactor, inhibiting the action of the catalyst and thereby reducing the SCR system effectiveness. High injection pressures are one way of dealing with the problem of insufficient atomization of the urea mixture, but high injection pressures often result in over-penetration of the injector spray plume into the exhaust stream, causing the plume to impinge on the inner surface of the exhaust pipe opposite the injector. Over-penetration leads to inefficient use of the urea mixture and reduces the range over which the vehicle can operate with reduced NOx emissions. Like fuel for the vehicle, only a finite amount of aqueous urea can be carried and what is carried should be used efficiently to maximize vehicle range and reduce the need for frequent fill ups of the reagent.
Additionally, aqueous urea is a poor lubricant. This characteristic adversely affects moving parts within the injector and requires that special fits, clearances and tolerances be employed between relatively moving parts within an injector.